nothing of these predictions, actually found the companion in the very
place assigned to it. In 1896 the companion of Procyon was discovered
by Professor Schaeberle at the Lick Observatory.

Now, by the refined parallax determinations of Gill at the Cape, we
know that of Sirius to be 0".38. From this it has been calculated that
the mass of Sirius equals two of our suns, and its intrinsic
brightness equals twenty suns; but the companion, having a mass equal
to our sun, has only a five-hundredth part of the sun's brightness.

_Spectroscopic Binaries_.--On measuring the velocity of a star in the
line of sight at frequent intervals, periodic variations have been
found, leading to a belief in motion round an invisible
companion. Vogel, in 1889, discovered this in the case of Spica (alpha
Virginis), whose period is 4d. 0h. 19m., and the diameter of whose
orbit is six million miles. Great numbers of binaries of this type
have since then been discovered, all of short period.

Also, in 1889, Pickering found that at regular intervals of fifty-two
days the lines in the spectrum of zeta of the Great Bear are
duplicated, indicating a relative velocity, equal to one hundred miles
a second, of two components revolving round each other, of which that
apparently single star must be composed.

It would be interesting, no doubt, to follow in detail the
accumulating knowledge about the distances, proper motions, and orbits
of the stars; but this must be done elsewhere. Enough has been said to
show how results are accumulating which must in time unfold to us the
various stellar systems and their mutual relationships.